1. Field of the Invention
The present invention relates to a method of cleaning a substrate and to a method of manufacturing a semiconductor device and, more particularly, to a method of cleaning a substrate and to a method of manufacturing a semiconductor device removing contaminants after working a surface of a substrate by chemicals etc., or treating the surface of the substrate by the chemicals, etc. prior to film formation.
2. Description of the Prior Art
In recent years, the prior art has mainly used a mixed solution consisting of sulfuric acid, hydrochloric acid, ammonium, hydrogen peroxide, etc. to clean a silicon wafer. Since metals and organic matters are removed, this cleaning method is effective.
A cleaning method using various steps to manufacture semiconductors in the prior art will now be explained with reference to FIGS. 1A and 1B which are sectional views showing steps of cleaning a surface of a semiconductor substrate 1 after trench grooves 3 have been formed in the semiconductor substrate 1.
As shown in FIG. 1A, the trench grooves 3 are first formed in the semiconductor substrate 1 using a resist film 2 as a mask. Then, the resist film 2 is removed by a resist removing liquid and, as shown in FIG. 1B, the semiconductor substrate 1 in which the trench grooves 3 are formed is exposed to a liquid such as sulfuric acid to remove contaminants such as resist removing liquid.
In the second step, as shown in FIG. 2A, a writing pattern 13 is first formed on an underlying insulating film 12 on a semiconductor substrate 11 by using the resist film 14 as a mask. Then, as shown in FIG. 2B, the resist film 14 is removed by the resist moving liquid. Then, as shown in FIG. 2C, an insulating film 15 is formed by the CVD method to cover the wiring 13 formed on the underlying insulating film 12.
In the third step, as shown in FIG. 3A, a surface of the underlying insulating film 22 on semiconductor substrate 21 is first cleaned by a liquid, e.g. sulfuric acid, etc. Then, as shown in FIG. 3B, a silicon containing insulating film 23 is deposited on the underlying insulating film 22 by the thermal CVD method using a reaction gas including TEOS (for example, N2 is employed as the carries gas) and ozone (which is included in the 02 at the predetermined concentration).
In the case of FIG. 1A and FIG. 1B, in recent years, with the progress of miniaturization of semiconductors, the opening of such a groove has become narrower and thus the ratio of depth to width of opening of the groove 3 (this ratio is called an aspect ratio) has become higher. Such an aspect ratio, at most, is five to eight. In the case of a groove which has a narrow opening width but a deep depth, in the chemical processing shown in FIG. 1B, it becomes difficult for the chemicals to reach the bottom of groove 3 and then difficult for the chemicals to be replaced by a pure water wash once the chemicals have entered into the groove. Thus, it is extremely difficult to completely clean the bottom and the side surfaces of the groove 3.
In the case of FIGS. 2A-2C, when the insulating film 15 is formed by the CVD method to cover the wiring 13 after surface treatment, it is difficult to deposit the insulating film 15 between the wire portions. As a result, as shown in FIG. 2C, deep hollows are sometimes produced between the wirings 13. If a conductive film is used to form an additional layer of wiring on insulating film 15, the conductive film cannot be satisfactorily deposited on such concave portions and the conductive film entering into the concave portions cannot be removed. If chemicals such as EKC (Trade name of EKC Company) is employed as the resist removing liquid, the growth rate of the SiO2 film 15 is extremely slow on the concave surface portions when the film is formed by the CVD method, after the pre-treatment, so that the concave portions cannot be covered evenly. The cause of this problem, though not clearly analyzed, is perhaps that the EKC cannot be completely removed from the surface of the underlying insulting film 12 and the remaining EKC has an adverse influence upon the growth rate of the Si02 film 15.
In the case of FIGS. 3A and 3B, there has been the problem that, when the film is formed on the insulating film 22, for example, a silicon thermal oxide film formed on the semiconductor substrate 21 by the thermal CVD method using a reaction gas including TEOS, the film forming rate is extremely slow on the insulating film 22 as compared with the case where the film is formed directly on the silicon substrate.
As the diameter of the wafer is enlarged from 200 mm to 300 mm, consumption of the chemicals is increased more and more. As a consequence, not only is the cost increased but also the drainage step takes an extreme amount of time, which creates environmental problems.
It is an object of the present invention to provide a substrate cleaning method capable of cleaning the interior of grooves having a narrow width and a deep depth, while reducing consumption of chemicals, and completely removing contaminants such as resist removing liquid, and forming a film on an insulating film with a desirable film forming rate.
In the present invention, a surface of a substrate is cleaned by use of a vapor including at least one of a vapor of sulfuric acid, a vapor of hydrochloric acid, a vapor of nitric acid and a vapor of chlorosulfonic acid (SO2Cl(OH)).
Since the chemicals are employed in vapor phase, consumption of the chemicals can be greatly reduced as compared to the case where a liquid is employed.
In addition, since a vapor of the chemicals is employed, the chemicals are in molecular form so that the chemical can enter into a groove with a narrow width and a deep depth. Accordingly, bottoms and side walls of grooves having an opening width of less than 0.3 xcexcm and a deep depth can be completely cleaned.
Further, as has been experimentally demonstrated contaminants which are hard to remove, for example, resist removing liquids, on the insulating film are completely removed by using a liquid or vapor of chlorosulfonic acid.
As also confirmed experimentally, when an insulating film is formed on the underlying insulating film by the thermal CVD method, substantially the same growth rate as that in the case where the film is formed directly on the semiconductor substrate is achieved, especially when using a liquid or vapor of chlorosulfonic acid.